Allergic diseases such as anaphylaxis and asthma affect ~10% of Americans, and can prove fatal in children under the age of 10. Importantly, these diseases accounts for >500,000 hospitalizations annually with billions of dollars in healthcare cost. Intense research over the past 30 years has increased our understanding of the pathogenesis of asthma and other allergic diseases. It has been well established that immunoglobulin E (IgE) and mast cells play critical roles in the pathogenesis of allergic disorders. The aggregation of IgE receptor (Fc?RI) on mast cells by IgE and allergen/antigen results in the rapid release of inflammatory mediators such as histamine and leukotrienes by a process termed as degranulation. Mast cells also release copious amounts of cytokines and chemokines, which result in the influx of other immune cells such as neutrophils, thus amplifying the allergic response. It has been recently accepted that G protein coupled receptors (GPCR) expressed on mast cells potentiate IgE-induced responses. Mast cells express the GPCR for the complement component C3a (C3aR) and mast cell-specific C3aR activation enhance IgE induced passive cutaneous anaphylaxis (PCA) and airway hyperresponsiveness (AHR) in asthma. C3aR has a class I PSD-95/Dlg/Zo1 (PDZ) binding motif in its carboxyl-terminus. Na+/H+ exchanger regulatory factors (NHERF1-4) are members of the class I family of PDZ domain proteins that bind several GPCR and regulate their trafficking and signaling. Recently, I demonstrated that NHERF1 promotes degranulation and chemokine production in C3a activated mast cells. However, NHERF1 did not interact with or affect the trafficking of C3aR. Thus, NHERF1 probably utilizes a unique and an undetermined pathway to regulate C3aR signaling in human mast cells. Moreover, whether NHERF1 regulates C3a-mediated mast cell responses in vivo, remains to be evaluated. This proposal builds up on my recent observations and aims at identifying the signaling pathway through which NHERF1 regulates human mast cell degranulation to C3a in vitro (Aim 1; mentored phase). In the independent phase, I will examine the role of NHERF1 in regulating mast cell mediated in vivo responses such as PCA (Aim 2) and AHR and lung inflammation in a mouse model of asthma (Aim 3). The completion of the proposed studies will help define the direct axis of NHERF1 in mast cells and allergic responses in vivo and will provide insights into the future development of NHERF1 antagonists that can target not only anaphylaxis and asthma but also other mast cell mediated allergic diseases. My mentoring team will consist of Dr. Hydar Ali (primary mentor), Dr. John D. Lambris (co-mentor) and Drs. Bruce Freedman, Raynold A. Panettieri, and Taku Kambayashi (Scientific Advisory Committee), all faculty members at the University of Pennsylvania (Penn) and renowned experts in their respective fields of investigation. I will take advantage of the intellectual strength and academic track record of my mentors and scientific advisory committee members, and the robust availability of expertise, facilities, and resources offered at Penn to accomplish this proposed training program.